Variations In The Isotopic Composition Of Near-Surface Water Vapour Over The Daily And Seasonal Cycle In The Eastern Mediterranean

Stable isotopes of water are extensively used in the tracking of the global water cycle, in evapotranspiration studies and paleo-climate reconstruction. Variations in isotopic ratios of water and water vapour are governed by isotopic fractionation due to phase changes and diffusion during evaporation, cloud evolution and rain out. Tracing these variations, in turn, can be exploited in environmental sciences for the reconstruction of climate conditions and variability. Using a long-term isotopic record of near surface water vapour in our lab in Rehovot we recently showed first, the importance of vertical mixing for the observed large seasonal cycle; and second, the presence of a long-term secular decreasing trend in the data. It was suspected that this reflected a small change in sampling timing that gave increasing weight to early afternoon well-mixed (and more depleted) PBL water vapour. This was tested by hourly sampling of atmospheric water vapour, over 17 different 24 h periods, at two different altitudes (~70 m asl, and ~110 m asl), between 23 June 2008 and 27 January 2009, accompanied by in situ relative humidity (RH) and temperature (T) recordings. On three selected 24 hr cycles a Lagrangian moisture source diagnostic was run which yielded calculated back trajectories of air masses arriving at Rehovot and provided averaged meteorological parameters (sea surface temperature, SST, relative humidity, RH, and wind speed at 10 m) over basins providing moisture to the sampling site during the collection period. The hourly diurnal sampling revealed a weak and variable diurnal cycle in water vapour isotopic values. On average, samples taken at night showed an almost perfect congruence with those taken during the day, as did those taken at the high elevation with respect to those taken at the very near surface. These findings clearly show that the average isotopic variations on the diurnal time scale could not have produced the observed long-term isotopic trend. Several regional parameters such as SST and 10 m wind speed also correlated well with observed isotopic measurements, and suggest some effect of environmental conditions in the source region. However, the strongest effect we observed was likely the correlation of vapour isotopic composition with RH that is especially prominent in dry days, when RH<60%. We therefore hypothesize that the long-term trend, observed in the isotopic composition of near-surface atmospheric water vapour, reflects a decreasing trend in RH. A long-term humidity records in a nearby meteorological station support this hypothesis.